1. Field of the Invention
This invention relates to a process for the vapor phase nitration of aromatic compounds. More particularly, this invention relates to a process for the vapor phase nitration of aromatic compounds in the presence of a nitration promoting catalyst comprising the adduct of a Group 4b-Group 3b mixed oxide composition represented by the empirical formula: EQU (M.sup.1.sub.a M.sup.2.sub.b O.sub.c).sub.x (NO.sub.2).sub.y
wherein M.sup.1 is at least one element selected from Group 4b of the Periodic Table of the Elements, M.sup.2 is at least one element selected from Group 3b of the Periodic Table of the Elements, a is 1, b is 0 to 20, c is a number taken to satisfy the average valences of M.sup.1 and M.sup.2 in the oxidation states in which they exist in the composition, x is 1, and y is 0 to c, and a catalytically effective amount of sulfur trioxide. Such catalysts are characterized by exhibiting a para/ortho isomer distribution of al least about 2/1 during the nitration of monosubstituted aromatic compounds having an ortho-para orientation substituent, especially chlorobenzene.
Nitroaromatic compounds find use as solvents, explosives, dyes, perfumes, and analytical reagents, and are important as intermediates in organic synthesis. As an example, nitroaromatic compounds are convertible by reduction into primary amines, which in turn, are valuable intermediates in the synthesis of dyes, pharmaceuticals, photographic developers, antioxidants, and gum inhibitors.
2. Description of the Prior Art
Nitroaromatic compounds are currently produced primarily via liquid phase reactions employing mixed acids. A sulfuric acid/nitric acid mixture is the most commonly employed industrial nitrating agent. Other mixed acids for nitration of aromatic compounds are acetic acid/nitric acid mixtures as described, for example, in U.S. Pat. No. 3,180,900. In U.S. Pat. No. 3,928,476, the latter type nitration is conducted over silica-alumina or alumina supports.
Vapor phase nitration of aromatic compounds is also known in the art. The vapor phase nitration of benzene and toluene at temperatures ranging from about 275.degree. C. to about 310.degree. C. is described in McKee and Wilhelm, Industrial and Engineering Chemistry, 28 (6), 662-667 (1936) and U.S. Pat. No. 2,109,873. McKee and Wilhelm catalyze their reaction with silica gel, with best results being reported by the use of 14 mesh material. Bauxite and alumina were reported to be ineffective as catalysts in the vapor phase nitration of benzene.
In U.S. Pat. No. 4,107,220, the vapor phase nitration of chlorobenzene in the presence of molecular sieve catalysts having a pore size varying from about 5 .ANG. to about 10 .ANG. as a means for controlling the para-to-ortho isomer distribution of nitrochlorobenzene is described. A suitable temperature range was reported to be from about 190.degree. C. to about 290.degree. C.
U.S. Pat. No. 4,347,389 describes a process for the vapor phase nitration of aromatic compounds. The process comprises contacting the aromatic compound with a nitrating agent in the presence of nitration promoting catalyst comprising a phosphorus-vanadium-oxygen complex.
More recently, in U.S. Pat. No. 4,415,744, a process is described for the vapor phase nitration of aromatic compounds in the presence of a specific catalyst composition. In this process, aromatic compounds are contacted in the vapor phase with a nitrating agent in the presence of a nitration promoting catalyst which comprises the adduct of:
(a) an alumina-silica-metal oxide combination represented by the formula: EQU (Al.sub.2 O.sub.3).sub.a (SiO.sub.2).sub.b (M.sub.2/n O).sub.c
wherein M is a metal cation selected from the group consisting of the lanthanides or rare earths, Groups 1b, 2b, 5b, 6B, 7b, and 8 of the Periodic Table of the Elements, and mixtures thereof, and a, b, and c represent weight percent of the Al.sub.2 O.sub.3 SiO.sub.2 and M.sub.2/n O components, respectively, in the alumina-silica-metal oxide combination, with a being 0 to 100, b being 0 to 100, and c being 0 to 50, and n represents an integer from 1 to 7 of the valence of the metal cation, with the proviso that the sum of (a+b) must be greater than 0, and
(b) a catalytically effective amount of sulfur trioxide.
Although the prior art processes are effective to provide the nitrated aromatic compounds, the choice of available catalysts is severely limited. In addition, the commercial utility of a catalytic process is highly dependent upon the cost of the catalyst system, the conversion of the reactant(s) and the selectivity and yield of the desired product(s). In many cases, a reduction in the cost of a catalyst system on the order of a few cents per kilogram or pound or a small increase in the yield of the desired product represents a tremendous commercial economical savings. Accordingly, research efforts are continually being made to define new or improved catalyst systems and methods and processes of making new and old catalyst systems to reduce the cost and/or upgrade the activity and selectivity of such catalyst systems in particular processes. The discovery of the process of the present invention, therefore, is believed to be a decided advance in the art.